The Hall effect is a known physical principle and is widely used for measuring magnetic fields, the essence thereof being made clear by referring to FIG. 1. A semiconductor body 1 is placed in an induction coil B and is traversed by a current I (I and B perpendicular).
The Hall voltage is the voltage VH appearing between the sides 2, 3 parallel to the current I circulating in semiconductor 1, when the latter is exposed to perpendicular magnetic induction B. It is due to the deviation by magnetic induction B of the electrons moving in semiconductor 1.
The law linking the different quantities is written: ##EQU1## in which n is the number of conduction electrons per cubic centimeter, t the thickness of the material and e the electron charge. Equation (1) consequently makes it possible to determine the induction Bz, to within an proportionality constant, if Iy and VHx are known.
In order to have a good sensitivity, a semiconductor material is sought with a minimum carrier density n and a thickness t.
On the basis of (1), it could be imagined that it would be sufficient to increase the polarizing current I to increase VH. However, there is unfortunately a practical limit to the increase of I, due to the resulting temperature rise. To extend the said limit, it is necessary to have a semiconductor with a good electrical conductivity.
As this electrical conductivity is expressed by the equation (2): EQU .sigma.=e.n..mu..sub.n ( 2)
in which .mu..sub.n is the mobility of the negative charges it is apparent that in order to have a good conductivity, it is necessary to use a semiconductor with a maximum mobility of the carriers .mu..sub.n.
The semiconductors which best satisfy this criterion are InSb with .mu.n=78,000 cm.sup.2 /Vs and InAs with 33,000 cm.sup.2 /Vs. For comparison, GaAs .mu.n=8500, InP .mu.n=4600, Ge .mu.n 3900 and Si .mu.n 1900.
Therefore, InSb is preferably used which, apart from the greater mobility, has the lowest melting point of 525.degree. C.
Another criteria which has become important of late through the development of new technology for the production of transducers is the aptitude therefore to permit mass and therefore economic production of large numbers of small transducers.
Thus, in summarizing, the main qualities required of such a technology are based on its possibilities:
(1) of using a material with a high mobility of the carriers .mu.n; PA1 (2) permitting the minimum semiconductor thickness t; PA1 (3) permitting the formation of very small transducers (few microns); PA1 (4) leading to the simultaneous production of a very large number of transducers. PA1 following vacuum deposition of the thin semiconductor film on the substrate, the active elements of the transducer are cut from said film by any known means, such as chemical etching or ionic bombardment, so as to produce these active elements in the form of narrow strips, particularly having a width less than 20 .mu.m; PA1 the substrate and active elements are then covered with an insulating SiO.sub.2 coating; PA1 the said coating is covered with a conductive metal coating with a thickness of approximately 1000 .ANG. and from a material such as NiCr or W, which has a good heat resistance; PA1 said heat-resistant metal coating is etched so as to form local heating resistors above the narrow semiconductor strips; PA1 passing an electric current into said heating resistors so as to obtain by the Joule effect the heating and then the melting and recrystallization of the semiconductor strips; PA1 by successive chemical etching operations, the local heating resistors above the semiconductor strips are removed and then the SiO.sub.2 coating is removed from the substrate except at the semiconductor strips; and PA1 a conductive coating is formed on the substrate for the contact points for supplying the transducer with polarizing current and for sampling the Hall voltage.
The oldest and presently most widely known technology for producing Hall effect detectors consists of using ingots of monocrystals of InSb or InAs, cutting them into slices, bonding these slices to a substrate, making the slices thinner by polishing and then etching the material to obtain the desired shape of the transducers.
With this process, condition 1 is very well satisfied because it is possible to use an almost perfect monocrystal. Condition 4 can be partly satisfied. However, it is very difficult to satisfy conditions 2 and 3, because only when great precautions are taken is it possible to achieve thicknesses of approximately 10 .mu.m, which still represents a high value. Apart from a sensitivity loss, this considerable thickness limits the fineness of the etching configuration and consequently the dimensions of the transducer which can be obtained (several dozen microns), whereas many applications require transducers of approximately 1 micron.
Confronted with this difficulty, numerous laboratories considered using thin film technology for depositing the semiconductor directly on the substrate. In this case, it is easy to satisfy conditions 2, 3, and 4, because it is easier to deposit a film with a thickness of a fraction of a micron than thick films.
However, in this case condition 1 is very difficult to satisfy. Thus, the material obtained by vacuum deposition is polycrystalline, i.e. it is formed by a large number of very small crystals, which significantly limit the mobility of the electrons, the mobility generally being below 1000 cm.sup.2 /Vs in the case of InSb. Under exceptional circumstances it can reach 10,000 cm.sup.2 /Vs using deposition conditions which are very difficult to control, while only having a poor reproducibility.
In order to improve this situation, numerous laboratories have envisaged crystallizing the semiconductor directly on to the substrate while remelting the metal after deposition, so as to obtain a larger grain size during solidification. The most interesting result published in this field is probably that of A. R. Clawson, "Thin solid film", no. 12, p. 291, 1972.
This author worked out a recrystallization method, which can be called the "hot wire method". It was in fact possible to obtain a mobility close to that of the monocrystal. This method, diagrammatically illustrated in FIG. 2 consists of placing a taut resistive wire 5 in the vicinity of the thin film 1 deposited on substrate 4, melting film 1 over a narrow width 6 (.about.1 mm) by passing an electric current through wire 5 and displacing the molten area 6 by moving wire 5 in front of substrate 1. The molten area 6 consequently separates the polycrystalline part 7 from the recrystallized part 8 of semiconductor 1.
Following this publication, this method had been taken up by several laboratories virtually without any modification. Good examples of attempted applications appear in the publications of Tetsu Oi et al., Japanese Journal of applied physics, vol. 17, no. 2, February 1978, pp. 407-412 and Nobuo Kotera et al, IEEE trans. on magnetics, vol MAG 15, no. 6, November 1979.
Although very interesting, this method suffers from a certain number of deficiencies, which explains why it has not passed beyond the laboratory stage.
The main difficulties encountered in practice are due to the fact that after recrystallization, there are very serious variations in the thickness of the film. These irregularities can even extend to holes at certain locations, which is readily apparent from the two aforementioned publications.
This disadvantage is particularly prejudicial in the case of real applications because, apart from the fact that it does not make it possible to produce devices with well controlled properties and geometries, it prevents any precise microlithography operations, and consequently the production of small transducers.
Thus, the authors have had to use a difficult process of polishing after recrystallization, which removes virtually all interest in the thin film technology.
Another disadvantage of the heating wire method is that it can lead to very severe thermal stresses in the substrate. In certain cases (e.g. detector on garnet for bubble stores) this is sufficient to break the substrate and prevent any use.
Another less serious, but nevertheless real difficulty of this method is that the thermal conductivity between the wire and the substrate has to be well controlled, which makes it necessary to work under helium and consequently makes the production process more difficult.